This invention relates to fractional and low ampere fuses using metallo-organic thin film ink as a fuse link and to a method of making these fuses.
Microfuses are used primarily in printed circuits and are required to be physically small. It is frequently necessary to provide fuses designed to interrupt surge currents in a very short period of time and at very small currents. For example, to limit potentially damaging surges in semiconductor devices, it is often necessary to have a low ampere fuse which interrupts in a time period of less than 0.001 seconds at ten times rated current, in order to limit the energy delivered to the components in series with the fuse.
Previous attempts to provide fuses operating in this range have utilized thin wires with a diameter of less than approximately 1 mil (1/1000 inch). The use of small diameter wire for fuse elements has a number of problems related to present manufacturing technology. One such problem is the high manufacturing cost for a thin wire microfuse. Since the fusible element has such a small diameter, the fusible element must be manually attached to the lead wires or end caps.
If solder and flux are used to attach the fusible wire element, it is difficult, in such a small device, to prevent the solder used to attach the wire ends from migrating down the wire during the manufacturing process. This solder migration causes a change in the fuse rating. In addition, the fuse rating may be changed when the external leads are soldered onto a printed circuit board since the heat generated in these processes can melt and reflow the solder inside the fuse. This also changes the fuse rating.
Another problem in manufacturing microfuses is the difficulty of coating the small diameter wire when encapsulating the fuse, as described in U.S. Pat. No. 4,612,529, so that arc quenching material, such as ceramic filler, surrounds the wire.
Methods of making fuses without wires as the fusible link are known. For example, McGalliard, U.S. Pat. No. 4,296,398, discusses forming a plurality of fuse elements by etch-resistant photography, silk screening, stamping or bonding. This technique, which is known as thick film printing, forms a layer of metal typically one half to one mil thick and suffers from several drawbacks. For example, the drying time for thick film prior to firing increases the manufacturing costs. Also, the width of the fusible element required to achieve low amperage ratings may be such that heat cannot properly be dissipated through the substrate during steady state operation. The typical thick film has limitation of thickness at about 0.5 mil thick, see for example, Ragan, U.S. Pat. No. 3,401,452. Thick film printing can achieve lines as narrow as 3 mil wide. Thus, it is not possible to produce fractional amp fuses with thick film elements due to the thickness and width limitations, i.e., the cross sectional area of the thick film is limited to 1.5 square mils, which will not melt at 1 amp or less.
Another method of making fuses is discussed in an article by Horiguchi, et al in IEEE Transactions On Parts Hybrid and Packaging, Volume PHP-13 No. 4, December, 1977. The fuse discussed comprises two layers, the first being an organic film, and the second, a nickel chromium film. This is a complicated manufacturing procedure in that evacuation is required for deposition of both for the organic layer and the metal layer and would add to the manufacturing cost. In this fuse construction, the organic film melts and damages the conductive layer, causing the fuse to open.